Isomerization in the 1,4,7,10,13,16-Hexaazacyclooctadecanecobalt(III) System [Co(hexacyclen)]3+, and Ion-Association Enhancement of the Unstable Facial Isomer

Abstract

Abstract The substitution reaction [Co(NH3)5(H2O)]3++hexacyclen·3H2SO4+6base+charcoal gives mer-[Co(hexacyclen)]3+ as the major product, and the equilibrium proportion of the fac isomer is <1%. The commercial hexacyclen contains linpen (the open chain analogue of hexacyclen, generated by an elimination reaction in the Richman-Atkins cyclization synthesis procedure), so that the synthesis of complex yields [Co(linpen)]3+ isomers as co-products which complicate the isolation of the pure [Co(hexacyclen)]3+ isomers. Procedures are now given for obtaining the mer and fac isomers pure. In particular, the unstable fac isomer is obtained by isomerization of the mer in base in the presence of the strongly associating anion PO43−. This yields the new equilibrium mixture mer: fac: hydrolysis products=71:18:11, wherein the fac proportion is enhanced twenty times, enabling the pure fac isomer to be obtained in quantity. For the base-catalyzed isomerization reaction fac → mer in NaOH solutions at 50 °C, procedures are given for following the reaction by analyses of samples using ion-exchange HPLC (Na2SO4 elution) and by spectrophotometry. The mechanism must involve intramolecular bond rupture, but the kinetic results are not consistent with a simple SN1CB mechanism, and a two-step process involving an intermediate is indicated.